In general, an induction motor includes a stator having a drive or excitation coil assembly, and a rotor having shorted coil assembly with a plurality of pole pairs. Typically, the stator is stationary while the rotor is rotatable with respect to the stator and is coupled to an output shaft for the motor. The motor generates torque due to the interaction between the stator magnetic field and the rotor magnetic field. The magnetic field of the rotor is induced from the stator by rotating the stator field at a somewhat different rate than the rotor rate. The difference frequency between the shaft (times the number of pole pairs of the motor) and the stator field frequency is known as the slip frequency. This slip frequency is seen by the shorted turns on the rotor. It is well known that the slip frequency strongly influences the basic machine characteristics such as torque constant and efficiency.
In the prior art, control of the induction motor may be achieved by employing a slip control loop, that is, a feedback loop around an induction motor that slaves the stator excitation frequency to be controlled in order to establish frequency difference above or below the shaft or rotor rotation frequency (times the number of motor pole pairs). That control may be achieved using a tachometer to convert shaft frequency to voltage. An offset voltage is added to represent the desired slip, and the resultant voltage is used to control a variable controlled oscillator (VCO). Since the slip frequency is typically only a few percent of the maximum motor frequency, this implementation is extremely sensitive to gain errors and non-linearities in both the tachometer and the VCO. Also, a tachometer is a relatively expensive transducer. In various alternative approaches in the prior art, mechanical differentials have been used in the slip loop, as well as circuits which estimate the slip from processing motor parameters and resolvers coupled to the output shaft. However, these approaches too pose substantial limitations. For example, the mechanical differential is a relatively costly device. Estimating slip from motor parameters is difficult due to the signal-to-noise ratio being relatively poor and the electrical model for the motor is temperature sensitive. In addition, the motor parameter processing approach is relatively ineffective at start, since there is no reflected voltage to sense. A resolver approach is in principal a satisfactory solution, however, in practice, the ruggedness and low maintenance characteristics of an induction motor cannot be matched to the use of slip rings. Another limitation in the above approaches is posed by the use of analog circuits which suffer from environmental conditions such as temperature. Furthermore, the response of the prior art and particularly the analog control systems is relatively slow.
U.S. Pat. Nos. 3,644,721 (Preiser), 3,568,022 (Domann) and 3,731,169 (Burgholte, et al.) illustrate another prior art induction motor controller approach using digital signal processing. In these systems, a pulse stream pulse repetition having frequency (prf) proportional to the output shaft is added (in frequency) to a pulse stream having a prf proportional to the desired slip frequency. The resultant pulse stream is used to drive an inverter network which in turn drives the stator coil assembly. However, the stator drive is generally a single, coarsely quantized, pulsed waveform and thus unsuited for driving a pulse-width modulated invertor for controlling an induction motor. The preferred drive signal for a pulse-width modulated inverter is a controlled amplitude sinewave at the desired stator frequency. For a multiphase motor multiphase controlled amplitude sinewave drives are required. Although the smoothing frequency tracking loop of Domann might reduce some of the waveform coarseness, the cost of that reduction is a substantial reduction in the system transient response.
It is an object of the present invention to provide a digital system for controlling the slip frequency of an induction motor.
Yet another object is to provide an induction motor slip control system having a relatively fast response to control variations in the slip frequency.
Still another object is to provide voltage or current references for PWM inverters for a fast response induction motor controller.
Another object is to provide multiple phase, controlled amplitude, sinusoidal-like waveforms for PWM inverters for a fast response induction motor controller.